kkhaven, do you have other furniture in the room(s)? I ask because 10' x 11.5' is plenty of room for a full size bed or a queen size bed. Maybe you should post your guest rooms here to see what furniture configuration would work best for each room. Has to be much less expensive than converting a 3 bedroom to a 2 bedroom. BTW, I agree with the consensus here - changing to a two bedroom from a 3 bedroom would drop your homes value tremendously.

@thinkdesignlive - thank you for your comments, to limit the cost we were thinking to live the garage doors and insulate on the inside only so it will limit the brick work. I really like your suggestion for the door (yes it will be on the yard for sure), it will make more sense as I cannot have only 2 bifolding doors. This is what I thought that I should contact @derdeb1234 - yes the budget is limit and we are not handy but we could increase to 10k. @tartanmeup - yes it is in GBP. @njmomma - Yes I will try to update the work. I still need to empty the garage, get rid of the spiders and insects, etc...

I agree with previous comment that if you want the space as a home office, you’ll need to close off one opening entirely - probably whichever is the bigger opening. For a couple reasons. As we’ve learned during quarantine, it‘s not good to have sounds easily transmitted when you are either trying to concentrate on your work or you are on a Zoom call. No one wants to hear in their home office the kids talking, the kitchen blender going, a nearby toilet flushing, or your SO on his/her call in the next room. Also for privacy - people walking by should not be able to see your computer screen showing, say, your bank statement, bills you are paying, confidential information from work, and the like. So close off one wall, and french doors on the other would be my recommendation. Furnishings-wise, I‘d remove the sofa and one or both of the armchairs - you need to add a desk, a spot to put the printer/scanner, and a couple file cabinets. File cabinets don’t have to be ugly, they make nicer-looking ones. Not too big file cabinets - try to be paperless as much as you can. Also a good task light for your desk, and an ergonomic desk chair. I bought an Aeron chair used - new they are $1300+; I paid $500 for a used one for what I think is the best desk chair on the market, but there are others out there for less.

Following up my earlier comment…if you pull the cubby into the middle of the space as a partition, there are lots of options for the desk…on the dark wall (monitors/wires will disappear/blend in); facing the back of the cubby (which could be trimmed out/painted/wall papered/decorated or covered in cork as someone suggested; or perpendicular to the cubby partition, especially if there is a window with a view on the wall we don’t see in your photos. Don’t worry about the monitors…arrange your desk in the position that feels most comfortable for you while you are working. The mudroom cubby area should be on the side closest to the door into the room, which I’m guessing is on the right since that is where you placed the cubbies. The good thing is you can move/tweak the placement of your desk and the cubbies until it feels just right for you! I would anchor the office space with a rug and place the chairs and the mirror (vertically) in the cubby space. You could do a gallery wall on the dark wall. I think it has a lot of potential and makes good use of the room!